When two people are approaching one another at an undefined rate, high five one another without stopping forward movement, and then continue to walk in opposite directions. A portmanteau of drive by and high five.

Yeah, I just gave Becky a five-by in the hall. It was awesome... High Five!

Similar in method to a Drive By. A person in a car or similar vehicle, while driving past a pedestrian, reaches out the window and gestures for a High Five.
The Five By is completed if the other individual outstretches their hand to meet with the driver's.
This act can be made more impressive by Five Bying a row of pedestrians, two vehicles participating in the Five By with one another, or alternatively the driver and passenger(s) in the same vehicle all Five Bying simultaneously.
As the latter is significantly more difficult, bonus points should be awarded upon successful completion.

I just Five By'd this chick on my way here. I think I shattered some bones in my wrist but it was so worth it.